I was wondering if any one had come across any interferences other than Iodide that would be a negative influence on the EPA 245-7 method for mercury analysis. I submitted a set of samples to a contract lab (because we are unable to analysis that low) and was expecting to see results around 50-100 ppt especially in two samples that were taken with very poor technique but all results came back <20ppt.

It's more likely that you've just experienced one of the great failings of the BrCl digestion that both EPA 245.7 and 1631E use. If your contract lab used the standard, room temperature option, the final result will NOT be total mercury but something I call "BrCl extractable." There can be a HUGE difference in the two results.

It is my humble opinion that neither method should have been promulgated at 136 because they more often than not fail to digest the mercury in wastewater, industrial waste, in fact any less-than-clean matrix. The EPA, infatuated with the possible low detection limits, ignored their own data and decided to add a lame "by the way" to the digestion sections of the methods.

Both 245.7 and 1631E should be at 141 (drinking water), but the EPA figured it would be detection limit overkill. So they fobbed the method off on us: big mistake. I've rattled EPA's cage over this for a couple of years, now. The silence of their replies has been deafening.

If you send me an email or address, I can send back a pdf or reprint of our article in WER that lays all of this out and shows comparison data that dramatically illustrates what I'm talking about.